\documentclass{article}
\def\nopagenumbers{\pagestyle{empty}}
\input pstfig
\usepackage{fp}
\begin{document}
$
\def\myr{2.2}
\def\smallnum{0.02}
\FPdiv\sixthpi{\FPpi}{6}
\FPcos\CosAlpha{\sixthpi}
\FPdiv\myR{\myr}{\CosAlpha}
\FPadd\myR{\myR}{\smallnum}
\thinline
\pscircle(0,0){\myr}
\pnode(0, 0){O}
\pnode(\myR; 0){A}
\pnode(\myR; 60){B}
\pnode(\myR; 120){C}
\pnode(\myR; 180){D}
\pnode(\myR; -120){E}
\pnode(\myR; -60){F}
%\psset{linewidth=1pt}
\thickline
\pspolygon(A)(B)(C)(D)(E)(F)
\thinline
\FPadd\myR{\myR}{\smallnum}
\psset{labelsep=0.2ex}
\pscircle(0,0){\myR}
\pcline(O)(B)
\Bput{R}
\pcline(O)(C)
\pcline(O)(0, \myr)
\nLput[offset=-1ex](O)(0, \myr){0.6}{r}
\pcline[linestyle=none](B)(C)
\Bput{a}
\psarc{<->}(O){0.5}{(B)}{(C)}
\uput{0.6}[78](O){\scriptstyle\alpha}
$
\end{document}
